No, it is unlocked at the very end of the post-game storyline missions.
You get the ability to evolve and you get to befriend palkia and so on
You first need to unlock the Surronded Sea area (a Sunflora mentions about going there and finding treasuere), there you'll find a Wonder Egg that will hatch into Manaphy, when Manaphy will return to you and mention Marine Resort. Then you can go to the Marowak Dojo and enter the Final Maze, Jirachi will be there at B23F.
You can find Wonder Gummis at Marine Resort. You can also get them as rewards for completing certain missions.
You have to get the Bottomless Sea Wonder Mail, and you must have unlocked Marine Resort. You might want to get a Golden Mask/Amber Tear, and you have to put up a good fight. See the Related Links for all the legendaries.
You win the game, get Cresselia and Manaphy to join your team, can recruit Palkia, access marine resort(lots of gummis, including wonder gummis and gunkis),and can evolve your partner and you after you recruit Palkia and have beaten all major bosses(Drowzee up to Darkrai).
You might need to get a secret slab or mystery part first.
First you have to get all seven treasures, get to master rank and then go to marine resort and beat him (I don't know if you have to beat him and I am doing the treasure hunt yet, so I don't know if you have to do something else). You can only explore the dungeons of the treasures by wonder mail. You can see the passwords in the Internet. hope this helped! :)
To unlock Serenity River:1PMJ +NS& 1M9C-FR@ NC3& +C34To unlock Shimmering Desert (where you can find Groudon): #&S6 NY2& YJN=1P57 F0MN MH7Y
There is a site posted on Google for Wonder Mail S Generators of Explorers Of Sky.
There is a wonder mail generator for explorers of sky here is the Link : http://apointlessplace.net/wms/wmgenerate.html
use this wonder mail HMQX5K4T6##5Y2N+HM09NPR@ also got to craggy coast or marine resort (not available if you are trying to feed baby manaphy)
www.wondermailscodes.com